The main advantage of in situ treatment is that it allows
ground water to be treated without being brought to the surface,
resulting in significant cost savings. In situ treatment,
however, generally requires longer time periods, and there is
less certainty about the uniformity of treatment because of the
variability in aquifer characteristics and because the efficacy
of the process is more difficult to verify.

Bioremediation techniques are destruction techniques directed
toward stimulating the microorganisms to grow and use the
contaminants as a food and energy source by creating a favorable
environment for the microorganisms. Generally, this means
providing some combination of oxygen, nutrients, and moisture,
and controlling the temperature and pH. Sometimes, microorganisms
adapted for degradation of the specific contaminants are applied
to enhance the process.

Biological processes are typically implemented at low cost.
Contaminants are destroyed and little to no residual treatment is
required. Some compounds, however, may be broken down into more
toxic by-products during the bioremediation process (e.g., TCE to
vinyl chloride). In in situ applications, these by-products may
be mobilized in ground water if no control techniques are used.
Typically, to address this issue, bioremediation will be
performed above a low permeability soil layer and with ground
water monitoring wells downgradient of the remediation area. This
type of treatment scheme requires aquifer and contaminant
characterization and may still require extracted ground water
treatment.

Although not all organic compounds are amenable to
biodegradation, bioremediation techniques have been successfully
used to remediate ground water contaminated by petroleum
hydrocarbons, solvents, pesticides, wood preservatives, and other
organic chemicals. Bioremediation has no expected effect on
inorganic contaminants.

The rate at which microorganisms degrade contaminants is
influenced by the specific contaminants present; temperature;
oxygen supply; nutrient supply; pH; the availability of the
contaminant to the microorganism (clay soils can adsorb
contaminants making them unavailable to the microorganisms); the
concentration of the contaminants (high concentrations may be
toxic to the microorganism); the presence of substances toxic to
the microorganism, e.g., mercury; or inhibitors to the metabolism
of the contaminant. These parameters are discussed in the
following paragraphs.

To ensure that oxygen is supplied at a rate sufficient
to maintain aerobic conditions, forced air, liquid oxygen, or
hydrogen peroxide injection can be used. The use of hydrogen
peroxide is limited because at high concentrations (above 100
ppm, 1,000 ppm with proper acclimation), it is toxic to
microorganisms. Also, hydrogen peroxide tends to decompose into
water and oxygen rapidly in the presence of some constituents,
thus reducing its effectiveness.

Anaerobic conditions may be used to degrade highly
chlorinated contaminants. This can be followed by aerobic
treatment to complete biodegradation of the partially
dechlorinated compounds as well as the other contaminants.

Nutrients required for cell growth are nitrogen,
phosphorous, potassium, sulfur, magnesium, calcium, manganese,
iron, zinc, and copper. If nutrients are not available in
sufficient amounts, microbial activity will stop. Nitrogen and
phosphorous are the nutrients most likely to be deficient in the
contaminated environment and thus are usually added to the
bioremediation system in a useable form (e.g., as ammonium for
nitrogen and as phosphate for phosphorous). Phosphates are
suspected to cause soil plugging as a result of their reaction
with minerals, such as iron and calcium. They form stable
precipitates that fill the pores in the soil and aquifer.

pH affects the solubility, and consequently the
availability, of many constituents of soil, which can affect
biological activity. Many metals that are potentially toxic to
microorganisms are insoluble at elevated pH; therefore, elevating
the pH of the treatment system can reduce the risk of poisoning
the microorganisms.

Temperature affects microbial activity in the
environment. The biodegradation rate will slow with decreasing
temperature; thus, in northern climates bioremediation may be
ineffective during part of the year unless it is carried out in a
climate-controlled facility. The microorganisms remain viable at
temperatures below freezing and will resume activity when the
temperature rises.

Provisions for heating the bioremediation site, such as use of
warm air injection, may speed up the remediation process. Too
high a temperature, however, can be detrimental to some
microorganisms, essentially sterilizing the aquifer.

Temperature also affects nonbiological losses of contaminants
mainly through the evaporation of contaminants at high
temperatures. The solubility of contaminants typically increases
with increasing temperature; however, some hydrocarbons are more
soluble at low temperatures than at high temperatures.
Additionally, oxygen solubility decreases with increasing
temperature.

Bioaugmentation involves the use of cultures that have
been specially bred for degradation of a variety of contaminants
and sometimes for survival under unusually severe environmental
conditions. Sometimes microorganisms from the remediation site
are collected, separately cultured, and returned to the site as a
means of rapidly increasing the microorganism population at the
site. Usually an attempt is made to isolate and accelerate the
growth of the population of natural microorganisms that
preferentially feed on the contaminants at the site. In some
situations different microorganisms may be added at different
stages of the remediation process because the contaminants change
in abundance as the degradation proceeds. USAF research, however,
has found no evidence that the use of non-native microorganisms
is beneficial in the situations tested.

Cometabolism, in which microorganisms growing on one
compound produce an enzyme that chemically transforms another
compound on which they cannot grow, has been observed to be
useful. In particular, microorganisms that degrade methane
(methanotrophic bacteria) have been found to produce enzymes that
can initiate the oxidation of a variety of carbon compounds.

Treatability or feasibility studies may be performed to
determine whether bioremediation would be effective in a given
situation. The extent of the study can vary depending on the
nature of the contaminants and the characteristics of the site.
For sites contaminated with common petroleum hydrocarbons (e.g.,
gasoline and/or other readily degradable compounds), it is
usually sufficient to examine representative samples for the
presence and level of an indigenous population of microbes,
nutrient levels, presence of microbial toxicants, and aquifer
characteristics.

Available in situ biological treatment technologies include enhanced biodegradation (nitrate and
oxygen enhancement with either air sparging or hydrogen peroxide
(H2O2)),
natural attenuation, and phytoremediation of organics. These
technologies are discussed in Section
4. Completed in situ
biological treatment projects for ground water, surface water,
and leachate are shown in Table 3-12 and
additional information on completed demonstration projects are
shown on the FRTR Web Site.

Implementation of biological treatment in vadose zone soils
differs from that of soils below the water table largely in the
mechanism of adding required supplemental materials, such as
oxygen and nutrients. For saturated soils, nutrients may be added
with and carried by reinjected ground water. Oxygen can be
provided by sparging or by adding chemical oxygen sources such as
hydrogen peroxide. Surface irrigation may be used for vadose zone
soils. Bioventing oxygenates vadose zone soils by drawing air
through soils using a network of vertical wells.